1. Field of the Invention
The present invention relates to methods and apparatus for encoding information in a signal by spectral notch modulation.
2. Description of the Related Art
Systems that transmit and receive digital signals to convey or collect information are being refined continually to improve signal throughput. A variety of factors affect the potential throughput of information in such systems, including: the signal bandwidth, the signal data rate, the transmit power, transmit beam directivity, transmission distances, receiver sensitivity, noise and interference levels, multipath reflections, the acceptable bit error rate, the modulation scheme, the error correction coding scheme, the amount of overhead and control signals required within the transmission protocol, and the sophistication of the transmitter and receiver equipment. Typically, information is encoded into a transmitted signal using one of variety of known modulation schemes, such as phase shift keying, frequency shift keying, quadrature amplitude modulation, amplitude modulation, or frequency modulation. By demodulating the signal at the receiver, the original information can be recovered.
Virtually all signaling systems can benefit from increased signal throughput. For example, in systems that primarily convey communication information such as data, voice, video, etc., it would be desirable to increase the proportion of the modulated signal that is devoted to the communication information while minimizing throughput lost due to the necessity of conveying overhead and control signals. Likewise, in systems that transmit navigation signals, such as time-of-arrival (TOA) ranging signals, it would be desirable to additionally convey communication information without having to transmit an excessive number of non-navigation messages that reduce the time available for navigation signals.
For example, U.S. Pat. Nos. 6,453,168 and 6,801,782, the disclosures of which are incorporated herein by reference in their entireties, disclose a state-of-the-art position location and communication system that provides accurate, reliable three-dimensional position determination of a handheld or portable, spread spectrum communication device within milliseconds without interruption of voice or data communications. Using spread spectrum waveforms and processing techniques, the system is capable of determining the position of a device to an accuracy of less than one meter and communications performance commensurate with the modulation and error correction coding that is employed in a severe multipath environment.
The position determining technique is based on transmitting and receiving TOA ranging packets that are used to measure the range between a master communication device and reference communication devices. Using trilateration, the measured ranges are transformed into the (x, y, z) location coordinates of the master communication device. The TOA ranging process uses a great deal of radio network bandwidth, leaving very little for communication of voice data or video information. As the TOA ranging update rate and number of references being used increase, the lack of air time for communications is compounded.
As described in the aforementioned patents, TOA ranging signals are exchanged between pairs of communication devices to determine the range between the devices. Once the exchange is complete, at least one additional message must be transmitted between the communication devices to supply information necessary to support the computation of the range and position. Theoretically, this data could be conveyed by adding data fields to the TOA waveform of the TOA ranging signals. However, this approach would add considerable complexity to the transmitter and receiver hardware as well as the waveform itself and would increase the duration of the TOA ranging signals. As with overhead signals that accompany the transmission of communication signals, it would be desirable to convey necessary support signals without reducing the time available for transmitting the primary signals of interest. More generally, it would be advantageous in many signaling systems to supplement the information throughput achievable with the primary modulation scheme with some form of additional signal throughput.